TCPD(8)TCPD(8)NAMEtcpd - access control facility for internet services
DESCRIPTION
The tcpd program can be set up to monitor incoming
requests for telnet, finger, ftp, exec, rsh, rlogin, tftp,
talk, comsat and other services that have a one-to-one
mapping onto executable files.
The program supports both 4.3BSD-style sockets and System
V.4-style TLI. Functionality may be limited when the pro-
tocol underneath TLI is not an internet protocol.
Operation is as follows: whenever a request for service
arrives, the inetd daemon is tricked into running the tcpd
program instead of the desired server. tcpd logs the
request and does some additional checks. When all is well,
tcpd runs the appropriate server program and goes away.
Optional features are: pattern-based access control,
client username lookups with the RFC 931 etc. protocol,
protection against hosts that pretend to have someone
elses host name, and protection against hosts that pretend
to have someone elses network address.
LOGGING
Connections that are monitored by tcpd are reported
through the syslog(3) facility. Each record contains a
time stamp, the client host name and the name of the
requested service. The information can be useful to
detect unwanted activities, especially when logfile infor-
mation from several hosts is merged.
In order to find out where your logs are going, examine
the syslog configuration file, usually /etc/syslog.conf.
ACCESS CONTROL
Optionally, tcpd supports a simple form of access control
that is based on pattern matching. The access-control
software provides hooks for the execution of shell com-
mands when a pattern fires. For details, see the
hosts_access(5) manual page.
HOST NAME VERIFICATION
The authentication scheme of some protocols (rlogin, rsh)
relies on host names. Some implementations believe the
host name that they get from any random name server; other
implementations are more careful but use a flawed algo-
rithm.
tcpd verifies the client host name that is returned by the
address->name DNS server by looking at the host name and
address that are returned by the name->address DNS server.
If any discrepancy is detected, tcpd concludes that it is
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TCPD(8)TCPD(8)
dealing with a host that pretends to have someone elses
host name.
If the sources are compiled with -DPARANOID, tcpd will
drop the connection in case of a host name/address mis-
match. Otherwise, the hostname can be matched with the
PARANOID wildcard, after which suitable action can be
taken.
HOST ADDRESS SPOOFING
Optionally, tcpd disables source-routing socket options on
every connection that it deals with. This will take care
of most attacks from hosts that pretend to have an address
that belongs to someone elses network. UDP services do not
benefit from this protection. This feature must be turned
on at compile time.
RFC 931
When RFC 931 etc. lookups are enabled (compile-time
option) tcpd will attempt to establish the name of the
client user. This will succeed only if the client host
runs an RFC 931-compliant daemon. Client user name
lookups will not work for datagram-oriented connections,
and may cause noticeable delays in the case of connections
from PCs.
EXAMPLES
The details of using tcpd depend on pathname information
that was compiled into the program.
EXAMPLE 1
This example applies when tcpd expects that the original
network daemons will be moved to an "other" place.
In order to monitor access to the finger service, move the
original finger daemon to the "other" place and install
tcpd in the place of the original finger daemon. No
changes are required to configuration files.
# mkdir /other/place
# mv /usr/etc/in.fingerd /other/place
# cp tcpd /usr/etc/in.fingerd
The example assumes that the network daemons live in
/usr/etc. On some systems, network daemons live in
/usr/sbin or in /usr/libexec, or have no `in.' prefix to
their name.
EXAMPLE 2
This example applies when tcpd expects that the network
daemons are left in their original place.
In order to monitor access to the finger service, perform
the following edits on the inetd configuration file
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TCPD(8)TCPD(8)
(usually /etc/inetd.conf or /etc/inet/inetd.conf):
finger stream tcp nowait nobody /usr/etc/in.fingerd in.fingerd
becomes:
finger stream tcp nowait nobody /some/where/tcpd in.fingerd
The example assumes that the network daemons live in
/usr/etc. On some systems, network daemons live in
/usr/sbin or in /usr/libexec, the daemons have no `in.'
prefix to their name, or there is no userid field in the
inetd configuration file.
Similar changes will be needed for the other services that
are to be covered by tcpd. Send a `kill -HUP' to the
inetd(8) process to make the changes effective. AIX users
may also have to execute the `inetimp' command.
EXAMPLE 3
In the case of daemons that do not live in a common direc-
tory ("secret" or otherwise), edit the inetd configuration
file so that it specifies an absolute path name for the
process name field. For example:
ntalk dgram udp wait root /some/where/tcpd /usr/local/lib/ntalkd
Only the last component (ntalkd) of the pathname will be
used for access control and logging.
BUGS
Some UDP (and RPC) daemons linger around for a while after
they have finished their work, in case another request
comes in. In the inetd configuration file these services
are registered with the wait option. Only the request that
started such a daemon will be logged.
The program does not work with RPC services over TCP.
These services are registered as rpc/tcp in the inetd con-
figuration file. The only non-trivial service that is
affected by this limitation is rexd, which is used by the
on(1) command. This is no great loss. On most systems,
rexd is less secure than a wildcard in /etc/hosts.equiv.
RPC broadcast requests (for example: rwall, rup, rusers)
always appear to come from the responding host. What hap-
pens is that the client broadcasts the request to all
portmap daemons on its network; each portmap daemon for-
wards the request to a local daemon. As far as the rwall
etc. daemons know, the request comes from the local host.
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TCPD(8)TCPD(8)FILES
The default locations of the host access control tables
are:
/etc/hosts.allow
/etc/hosts.deny
SEE ALSOhosts_access(5), format of the tcpd access control tables.
syslog.conf(5), format of the syslogd control file.
inetd.conf(5), format of the inetd control file.
AUTHORS
Wietse Venema (wietse@wzv.win.tue.nl),
Department of Mathematics and Computing Science,
Eindhoven University of Technology
Den Dolech 2, P.O. Box 513,
5600 MB Eindhoven, The Netherlands
MODIFIED BY BSDI
BSDI added the exec and nolog options. Please note that
any problems with these options should be addressed to
<problem@BSDI.COM>.
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